1. Field of the Invention
The present invention relates to a microphone device for use in a communication apparatus for a motorcycle, and more particularly to a type of a microphone device for close positioning relative to a user which is mounted on an operator's helmet or the like to transmit a message for example during driving operation of a motorcycle.
2. Description of the Background Art
FIG. 8 illustrates a microphone device of the above mentioned close talking type (hereinafter, referred to as "close talking microphone device) as an example. In the drawing, FIG. 8(A) is a front view of the close talking microphone device 100, wherein the side facing an operator is referred to as the front of the microphone device and the reverse side is referred to as a back thereof. FIG. 8(B) shows a cross sectional view of the microphone device and FIG. 8(C) shows a back view thereof. As apparent from FIG. 8(B), the close talking microphone device 100 has a front casing 101 and a back casing 102 combined with each other to form an internal space therebetween in which a microphone 103 is accommodated. Elastic supporting means 104 and 105 are arranged at each front and back of the microphone 103 to support the latter in a floating condition. On the front casing 101, there is formed a plurality of slots 106 to communicate a message transmitting voice therethrough. As will be seen from FIG. 8(C), however, no slot is formed on the back casing 102. This microphone device 100 is low sensitive to a remote source of a sound than the message transmitting voice and has such a characteristic as not to be susceptible to pick up an environmental noise.
When the message transmitting voice is transmitted by the microphone device 100 during a driving operation of a motorcycle, the environmental noise is susceptible to be picked up. The environmental noise includes a noise generated by a running operation and an outside noise. The noise in the running operation includes a contact noise between wheels and a road, a noise of an engine operation and a noise caused from a ram air. On the other hand, the outside noise includes other vehicle noise, which is generated by other vehicles running near by, especially side by side and a noise of road construction or the like.
In the motorcycle, a noise which is particularly an obstacle to the message transmission is a sideward noise such as other vehicle noise which is communicated from a direction of 90 degrees (sideward) with respect to an input direction of the message transmitting voice. To remove the sideward noise, it is necessary to employ the close talking microphone device which has such a directivity (directional characteristic) as to be low sensitive to the sideward noise. In this respect, it is effective for the microphone device to be provided with the directivity of substantially the shape of the number 8 as shown in FIG. 7.